1. Field of the Invention
The invention consists of improvements to rotationally symmetric dual reflector systems and symmetric cylindrical dual reflector systems. These antennas are used for the transmission and/or reception of electromagnetic waves. The antennas are used in many applications which include point to point links, telemetry and satellite communication.
2. Brief Description of Related Art
Dual reflector systems are commonly used in communication systems. They generally comprise a main reflector usually based on a parabolic shape and a sub-reflector, usually based on a hyperbolic or elliptical shape, and a feed. The systems with sub-reflectors using hyperbolic shapes are referred to as Cassegrain systems while the ones using elliptical shapes are referred to as Gregorian systems. In the transmitting mode, a feed is used to radiate energy towards the sub-reflector. The energy bounces off the sub-reflector towards the main reflector and then bounces again off this main reflector. In a receiving mode, the energy follows the reverse path.
Generally the description above and all of the description below apply to cylindrical geometries (where the cross-section of the geometry remains essentially constant) or rotationally symmetric geometries where each reflector is a surface of revolution. In both of these types there are many variations where the actual reflector shapes are cut from cylindrical or rotationally symmetric shapes. For example, it is common to base the reflector shapes on a portion of rotationally symmetric shapes so that the actual reflector has an elliptical rather than circular projection. In systems which are large in relation to the wavelength of the transmitted or received radiation, the reflector systems and shapes of the reflectors can be designed with the use of optical techniques.
When the dual reflector system is reduced in size, the sub-reflector may become small in relation to the wavelength of the received/transmitted radiation. Also, it is common to support the sub-reflector by attaching it to the feed via a dielectric support structure. Commonly, this is a tube or a rod or a partial cone. In this case the optical techniques used for the design of the bigger systems do not work very well. For the small reflectors more elaborate techniques which account for the near field effects of the sub-reflector, the support and the feed are used.
Common methods of analysis of such antenna systems are the Method of Moments and FDTD (Finite Difference Time Domain). It has been found by many authors that sub-reflector shapes other than those based on hyperbolas and ellipses work well. Furthermore, in early designs, the sub-reflectors were simple metallic plates and the sub-reflector-feed combinations were called xe2x80x9csplash platexe2x80x9d feeds. One of the major attractions of the geometries described above is the location of the feed. It protrudes through a hole inthe main reflector and is attached to the main reflector near its vertex. This allows the shortest possible paths from the transmitter and/or receiver which are usually housed behind the main reflector.
The major problem with the geometries described above is the blockage caused by the sub-reflector or sub-reflector-feed combination. This blockage can be easily seen when the antenna is operating as a receiver. The blockage mechanism is crudely described by the following: The radiation that hits the sub-reflector is reflected by it and does not reach the main reflector. However, the radiation that reaches the main reflector is reflected towards the sub-reflector which bounces it into the feed. Generally the blockage causes two undesirable effects to the antenna radiation pattern. The first is a reduction in the antenna""s on-axis gain and the second is an increase in the level of the side lobes. In particular, the side lobes close to the main beam (inner side lobes) can be greatly increased by the blockage.
Although the invention can be used for large antenna systems with small frequency bandwidths, it will be particularly useful in smaller antenna systems such as those which previously used xe2x80x9csplash platexe2x80x9d feeds. Many workers have studied these reflector systems. In particular, the invention can replace those described in U.S. Pat. Nos. 4,963,878, 6,020,859 and 5,959,590. The first two of these patents describe an evolution of inventions by Kildal. The third patent by Sanford et al. is an improvement on earlier inventions by Kildal. These patents describe various shaped sub-reflectors and main reflectors.
The intention of these above-described inventions is to improve the far-field pattern performance of the antenna system. These inventions improve the gain and far out side lobes of the antenna patterns. They also allow operation of the antenna in a dual polarization mode. U.S. Pat. No. 6,137,449, also by Kildal, describes a number of ways of improving the mechanical design of the antenna plus a method for producing a dual band antenna.
In modern antenna systems there is increasingly a requirement to produce 1) high gain antennas with 2) low inner side lobes, 3) low far side lobes and 4) low VSWR (Voltage Standing Wave Ratio). With this invention it is possible to produce a better compromise between all four requirements than could be done previously.
The invention described here differs greatly from earlier inventions U.S. Pat. Nos. 4,963,878, 6,020,859 and 6,137,449 by Kildal since these do not address the problem of blockage of the feed and do not place a hole in the sub-reflector. The invention in U.S. Pat. No. 5,959,590 by Sanford et al. partially addresses the blockage problem by producing a small sub-reflector but does not include a hole in the sub-reflector.
Other important differences between the prior art and the present invention is the use of a more elaborate feed aperture and a simpler dielectric support. In the inventions by Kildal and Sanford et al, a simple tube is used as a feed but an elaborate dielectric plug is typically used to support the sub-reflector. In this present invention one or more chokes on the feed aperture help to control the radiation from the feed. Typically the feed aperture will be approximately as large as the sub-reflector. This larger diameter feed aperture produces more control of the radiation from the feed-sub-reflector combination and allows more freedom in the design of the dielectric support. Another benefit is improved control of the VSWR (voltage standing wave ratio) measured in the feed.
Due to the complexity of the interactions between the antenna components, it is not possible to produce closed form formulae for their dimensions. Rather, the goal of the invention is to establish a general geometry from which specific designs can be found which meet the desired requirements for particular applications. The detailed dimensions of the components can only be found by utilizing a computer optimizer which controls an accurate computer analysis program. Nowadays, there are a number of software packages available with these capabilities.
The invention allows the minimization or elimination of the sub-reflector blockage effects. In its simplest form, the invention is the inclusion of a hole or opening in the sub-reflector. This allows some energy to travel directly to or from the feed sub-reflector combination and by-pass the main reflector. In general the radiation that passes through the hole will not be in phase with the radiation which travels via the path that includes the main reflector. Usually, the latter path is much longer. This is where careful design of the reflector system is required.
By appropriate design of all the components in the antenna system, it is possible to force the two paths to be different by approximately an integer number of wavelengths and therefore force the two signals to be in phase. The number of wavelengths difference in the path lengths determines the frequency bandwidth over which the hole produces an improved antenna pattern. Increases to the difference in path length decrease the frequency bandwidth. A larger bandwidth can be achieved by implementation of a device which slows the radiation which passes through the hole. One such device is a dielectric rod for rotationally symmetric geometries or a dielectric slab for cylindrical geometries.
The invention applies equally well to antennas based on rotationally symmetric components or on cylindrical components. There are a number of components and surfaces that can be used to control the relative amplitude and phase of the radiation through the hole and the radiation which bounces off the main reflector. These are the inner surface of the sub-reflector (the surface which faces the feed and main reflector), the outer surface of the sub-reflector which faces away from the main reflector and feed, the main reflector, the feed aperture and the dielectric piece which supports the sub-reflector. There are eight components to the antenna system. The first five are essential to the invention. The others may exist depending on the antenna requirements.
A main reflector.
A shaped feed aperture.
A shaped outer surface of the sub-reflector.
A shaped inner surface of the sub-reflector.
A hole or opening in the sub-reflector.
A device which supports the sub-reflector.
A device used to slow the hole radiation e.g. a dielectric rod or slab.
A radome.
The structure of feed sub-reflector combinations in rotationally symmetric antenna systems naturally produces a ring focus rather than a point focus. Thus, in the preferred embodiment, the main reflector is usually based not upon a paraboloid but on a surface of revolution of a half parabola whose axis is parallel to, but offset from, the axis of revolution. This shape will be referred to as a SROP (Surface of Revolution of an Offset Parabola).
In cylindrical antenna systems, the same principle applies. The main reflector is based on a parabola whose two halves are separated by some distance. For improved pattern control, the shape of the main reflector is often perturbed from the pure parabolic shape. Improved frequency bandwidth is achieved by the reduction of the difference between the path length of the radiation which passes through the hole and that of the radiation which bounces off the main reflector. This is achieved by choosing a main reflector with a small F/D (Focal length divided by Reflector Diameter) ratio.
In cylindrical geometries, the feed usually contains a parallel plate waveguide. Depending on the separation of the plates, this guide can support one or more polarizations. For rotationally symmetric geometries, the feed usually contains a circular waveguide but in some applications a coaxial waveguide transmitting and/or receiving the TE11 mode can be used. Around the mouth of these waveguides, one or more chokes are used to help control the radiation from the feed and the VSWR. Commonly, for the same reasons, transformer sections are also added to the waveguide.
The shape of the outer surface of the waveguide varies greatly from application to application. It is used to help control the shape of the radiation pattern of the energy that passes through the hole. In some narrow band applications, the surface may contain little or no shaping. For other applications, the surface can be shaped like a horn. The inner surface of the sub-reflector is used to control the relative amounts of radiation passing through the hole and between the feed and sub-reflector. It also helps control the VSWR seen in the feed waveguide. Like the outer surface, there are applications where the inner surface is very simple and other applications where the inner surface can resemble a stepped cone.
The size and length of the hole in the sub-reflector help control the amplitude and phase of the radiation through the hole. Usually a dielectric plug is used to reduce the size of the hole while still allowing the radiation to pass through the hole. The plug is also used for environmental reasons since it helps enclose the cavity between the feed and the sub-reflector. A convenient means of supporting the sub-reflector in rotationally symmetric antenna systems is to use a dielectric tube. The tube can be relatively thin while still producing a sturdy mechanical support for the sub-reflector. The tube is usually glued to the feed aperture and the sub-reflector.
In cylindrical systems, the supports can be integrated with the dielectric piece which fills the opening in the sub-reflector. In these geometries, the sub-reflector is actually made from two separate pieces which can be glued to the integrated dielectric piece which in turn is glued to the feed aperture. Dielectric rods and slabs are waveguiding structures which slow the wave. If one of these is used in the radiation path through the hole, the effective path difference between this radiation and the radiation which bounces off the main reflector is reduced. This results in an improved frequency bandwidth. The dielectric rod or slab can be integrated with the plug which fills the hole in the sub-reflector. This produces a sturdy mechanical arrangement. Radomes are required for most antenna systems.
There are many choices in shapes and location of the radome. Many times they are placed over the rim of the main reflector. Depending on the frequency of operation and the mechanical constraints on the radome materials and thickness, the radome can have a significant effect on the performance of the antenna. This is particularly true for the low side lobe, high frequency applications. Because of this, the effects of the radome must be included in the computer modeling of the antenna.
This invention possesses many other advantages and has other purposes which may be made more clearly apparent from a consideration of the forms in which it may be embodied. These forms are shown in the drawings forming a part of and accompanying the present specification. They will now be described in detail for purposes of illustrating the general principles of the invention. However, it is to be understood that the following detailed description and the accompanying drawings are not to be taken in a limiting sense.